The earlier sentences of that paragraph should explain it. First, if a SMC attacks alone it must defend alone. If it attacks with other units, it can add its FP to one unit (that it is stacked above to indicate that). Since a leader can't direct its own attack, in order to direct an attack it must be combined with another unit, they attack together (possibly with other units) and defend -- are attacked -- together (again, possibly with other units if you opponent so chooses).